The page width is defined like this in the liquid files:
page-width--xxxxxxx
If you are changing the width of pages, you probably also want to change other parts of the Dawn theme, like blog posts, etc…
For blog posts, and possibly others, edit these files and change
page-width--narrow --> page-width--wide
Affected filed:
Sections > main-page.liquid
Sections > page.liquid
Sections > main-article.liquid (multiple lines)
There might be more places in the many liquid files that need changing!